Fix race condition that caused intermittent spec failures in zadd

Updated specs to use a random jid when adding a job in the specs, rather than a fixed jid.  This ensures that member objects passed to zadd are unique (which they may not be otherwise, if Time.now.to_f is the same).  This issue was causing intermittent failures of a couple specs on JRuby.  Not sure why it wasn't showing up on MRI.

As a side note, upgrading to JRuby 1.7.9 fixes this issue - https://github.com/jruby/jruby/issues/1291 - which means that after this PR is merged in specs should run green on JRuby.